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25 Jan 2024

CUHK study estimates half of COVID-19 infections went unrecognised during the Omicron epidemic in Hong Kong

25 Jan 2024

The Chinese University of Hong Kong (CUHK)’s Faculty of Medicine (CU Medicine) conducted a study to estimate the proportion of unrecognised COVID-19 infections during the Omicron wave and the true total number of cases, both reported and unreported. The results showed that 33.6% of the total population in Hong Kong were infected between 1 January and 20 June 2022, corresponding to approximately 2.5 million people out of a total population of about 7.5 million. Among these infections, nearly one in two (16.2% out of 33.6%) were unrecognised. Study results have been published in the journal Emerging Infectious Diseases.

The study uses the seroprevalence of antibodies to the ORF8 antigen to infer the number of unrecognised infections

The Open Reading Frame 8 (ORF8) protein is unique to SARS-CoV-2. Therefore, the presence of ORF8 protein antibodies in blood samples is a reliable serologic marker of natural COVID-19 infection. In this study, blood samples were collected from 1,028 volunteers who self-reported as not having been infected. Based on the seroprevalence of ORF8 antigen antibodies relative to the number of reported cases at different times, the research team used a statistical modelling method to estimate the actual numbers of symptomatic, asymptomatic and unreported asymptomatic infections, the attack rate and the reproduction number.

Testing for ORF8 seroprevalence proves promising for monitoring COVID-19 outbreaks

During the Omicron wave, the Hong Kong government launched an online system for people to self-report positive cases identified by self-administrated rapid antigen tests (RAT). However, one main shortcoming of RATs was that a large number of infections likely went untested and unreported because of a high proportion of asymptomatic or mild cases. Compared with RATs, testing for ORF8 seroprevalence is able to identify those asymptomatic or mild cases and is a promising approach for epidemiology study to find out the proportion of the population that have been infected, as supported by the study findings.

Professor David Hui Shu-cheong, the corresponding author of the study and Director at S.H. Ho Research Centre for Infectious Diseases at CU Medicine, stated, “Our study findings highlight the usefulness of testing for ORF8 seroprevalence among efforts to monitor COVID-19 outbreaks, especially for emerging new variants of concern.”

Professor Marc Chong Ka-chun, also the corresponding author of the study and Assistant Professor at The Jockey Club School of Public Health and Primary Care of CU Medicine, added, “Public health agencies need to take into account the potential for a substantial undercount of the actual number of infections when considering the commitment of resources to prevent and control outbreaks of emerging infectious diseases.”

The research team members included Dr Zhao Shi, a Postdoctoral Fellow at The Jockey Club School of Public Health and Primary Care at CU Medicine at the time of the study; and Professor Chris Mok Ka-pun, Assistant Professor, Dr Tang Yun-sang, Postdoctoral Fellow, and Miss Chen Chunke and Mr Sun Yuanxin, Research Assistants, all from The Jockey Club School of Public Health and Primary Care at CU Medicine.